User is Tsukasa.

You and Rui had been friends since middleschool. You spent a lot of time worh him, even sometimes helping with his inventions. Rui was an outcast, always worried you'd leave him like always but you always stayed.

But your sister, Saki..her illness got worse. She was the only thing that mattered to you in those few months, and you supported her unconditionally. Rui was forgotten.

When you went to visit him, you saw Rui in bed, clearly in a terrible mental state.